Ocular astigmatism (referred to hereinafter simply as “astigmatism”) is a phenomenon associated with variation of the eye's optical power as a function of meridian. For example, one eye may require a first dioptric correction of one diopter along one meridian, and a second, different correction or no correction along another meridian. The astigmatism typically arises due to asymmetry of the cornea or tilting of the crystal lens in the eye. When an eye requires a non-varying maximum correction along a first meridian and a non-varying minimum correction along a second meridian, the meridians being separated by an angle of 90 degrees, this is called “regular astigmatism.” When the angle is not 90 degrees, or the necessary correction varies along a meridian or there are more than two meridians, this is called “irregular astigmatism.”
Ophthalmic lenses, for example contact lenses and intraocular lenses (IOLs), for correcting refractive abnormalities of the eye associated with astigmatism typically have a toric optical zone (also referred to herein simply as a toric zone). Such lenses are commonly referred to in the industry as “tonic lenses.” The toric optical zone provides cylindrical correction to compensate for the astigmatism. Since astigmatism that requires vision correction is usually associated with other refractive abnormalities, such as myopia (nearsightedness) or hypermetropia (farsightedness), toric lenses are generally prescribed also with a spherical correction to correct myopic astigmatism or hypermetropic astigmatism. The toric surface may be formed on either the posterior lens surface (back surface toric lens) or in the anterior lens surface (front surface toric lens).
Toric contact lenses have several complications associated therewith. Whereas conventional spherical contact lenses may freely rotate on the eye, toric contact lenses have some type of ballast to inhibit rotation of the lens on the eye so that the cylindrical axis of the toric optical zone remains generally aligned with the eye's astigmatic axis. For example, to provide such ballasting, one or more sections of the lens periphery may be thicker (or thinner) than other sections. Examples of ballasts may include prism ballasts and peri ballasts.
Toric contact lenses are manufactured with a selected relationship (commonly referred as angular offset or simply offset) between the cylindrical axis of the toric optical zone and the orientation of the ballast to correct astigmatism at various angular orientations. This relationship is expressed as the number of degrees rotation that the cylindrical axis is offset from the orientation axis of the ballast feature(s). Accordingly, toric contact lens prescriptions specify this offset, with toric lenses generally being offered by contact lens manufacturers in 5-degree or 10-degree increments ranging from 5 degrees (or 10 degrees) to 180 degrees.
In summary, a prescription for such toric contact lenses will typically specify spherical correction (i.e., dioptric power), cylindrical correction (i.e., dioptric power) and axis offset (i.e., angular rotation) to define the optical correction.
It will be appreciated that toric contact lenses are much more complicated for a practitioner to fit than spherical contact lenses which have only a spherical correction. Although toric lenses include a ballast to inhibit rotation, a toric contact lens may still have too much rotational instability for some patients' eyes. Furthermore, if the cylindrical axis of the lens is misaligned with the eye's axis of astigmatism, the astigmatism is not fully corrected, and, in fact, vision may be further impaired. Many astigmatic patients cannot be fitted for contact lenses for these reasons.
A further disadvantage of toric contact lenses is that a manufacturer has to make, inventory, and supply a very large number of lens units. In comparison with spherical contact lenses which include only a spherical optical correction, for toric contact lenses, the manufacturer must make, for each spherical correction, a range of cylindrical corrections, with each being offered at a range of axis offsets from 5 degrees (or 10 degrees) to 180 degrees. From a practitioner standpoint, the practitioner either needs to maintain a large inventory of toric contact lenses in his/her trial fitting sets for trial testing on a patient in his/her office, or trial fit a patient with a lens that does not have the correct optical correction. Since toric contact lenses are more expensive to manufacture and inventory, their cost is substantially higher than spherical contact lenses.
Intraocular lenses (IOLs) replace the natural crystalline lens of a patient. If the patient is myopic, an IOL including myopic correction may be implanted. If the patient has astigmatism, the ophthalmic professional may employ a toric IOL, including a toric zone having a cylindrical correction, to compensate for the astigmatism.
During surgical implantation of a toric IOL, the cylindrical axis of the toric IOL must be aligned with the eye's axis of astigmatism, otherwise the astigmatism is not fully corrected; in fact, vision may be impaired more than if a spherical IOL had been used. However, even if the IOL is correctly aligned in the eye to correct the astigmatism during the surgical procedure, it is not uncommon for the lens to rotate during the post surgical healing phase, resulting in angular misalignment. For this reason, many ophthalmic professionals may be hesitant to implant a toric IOL for risk of deteriorating vision quality rather than improving it as intended.
Surgeons may only try to correct 1.25 D (diopters) of astigmatism or greater because the error in the system may not allow a benefit in vision to be demonstrated for astigmatism lower than 1.25 D. For patients having less than 1.25 D of astigmatism, while the patient could benefit visually from correction, the lack of accuracy and precision in aligning the toric lens in surgery may preclude surgeons from attempting to correct these lower amounts.